1. Field of the Invention
The present invention relates to a reference electrode which imparts a standard potential to a chemically sensitive electrode of potentiometric type.
2. Description of the Prior Art
Chemically sensitive electrodes are roughly classified into two types. A first type is the potentiometric electrode for measuring the concentration of a particular chemical substance in a test solution by making use of a potential gradient at the interface between the electrode and the solution. As those belonging to this type, there can be mentioned pH sensitive electrodes and ion sensitive electrodes which are sensitive to such ions as Na.sup.+, K.sup.+ and Ca.sup.2+. A second type is the amperometric electrode which utilizes electric currents produced by oxidation-reduction reaction taking place on the electrode surface. As those belonging to this type, there can be mentioned oxygen electrodes and hydrogen peroxide electrodes. The potentiometric measuring system requires a reference electrode to be associated with a chemically sensitive electrode. A reference electrode is an electrode which is not sensitive to the various components in a test solution and which imparts a standard potential to a chemically sensitive electrode.
So-called liquid-junction type reference electrodes, such as calomel electrodes and silver-silver chloride electrodes, have heretofore been used as reference electrodes. As an example of a liquid-junction type reference electrode, the basic construction of a silver-silver chloride electrode is shown in FIG. 4. This silver-silver chloride electrode comprises a silver wire 1 having its surface chlorinated, said silver wire being received in a container 2 together with an internal solution 5 containing chloride ions of given concentration. The container 2 has a liquid-junction port 3 and an internal solution supply port 4. The liquid-junction port is provided for forming a liquid junction between the internal solution and the test solution, the liquid junction region being usually in the form of a pin hole, porous ceramic or the like. The potential at the silver-silver chloride wire changes with the chloride ion concentration of the internal solution, as indicated by equation (1). ##EQU1## where F stands for the Faraday constant, R the gas constant, T the absolute temperature, and E.sub.o a constant. The internal solution and the external test solution exchange, though gradually, through the liquid junction port and hence the chloride ion concentration of the internal solution changes. As a result, a fresh internal solution must be supplied at times through the supply port 4. Because of such basic construction, the liquid-junction type reference electrode has the following problems. (A) The electrode, particularly the liquid junction region, should not be allowed to dry during storage. Supply of internal solution and maintenance are troublesome. (B) It sometimes occurs that the internal solution leaks into the test solution through the liquid junction port, thus contaminating the test solution. (C) Reduction in size can hardly be attained.
On the other hand, a reference electrode FET (REFFET) based on an ion sensitive field effect transistor, which is a dry type reference electrode used in place of the liquid-junction type reference electrode, is known (Japanese Patent Publication No. 58-25221, Japanese Patent Application Laid-Open Specification Nos. 55-12480, 55-101852, 55-101853, 55-164348, and 56-100350, and U.S. Pat. No. 4,269,682). This reference electrode comprises an ion sensitive field effect transistor with its gate surface coated with a membrane of an ion insensitive hydrophobic organic high polymer, the potential at said reference electrode being determined by the potential gradient at the interface between the hydrophobic high polymer membrane and the solution. And in order for this reference electrode to function effectively as such, it is necessary that the potential not be influenced by the pH and total ion concentration of the solution. To this end, the following conditions should be satisfied. (1) Said high polymer membrane should be free of pinholes which would allow water and ions to pass therethrough. (2) Said high polymer membrane should have no or little, if any, amount of functional groups, such as OH, NH.sub.2, &gt;C.dbd.O and &gt;NH, which are capable of combining with metal ions to form complex compounds. (3) The thickness of said high polymer membrane should be within the range of 300 to 10,000 angstroms. However, without forming pinholes it would be difficult to form a 300 to 10,000-angstrom thick membrane on the surface of an ion sensitive field effect transistor. Further, if a hydrophobic high polymer membrane is kept in contact with air or water for a long time, a problem arises that the surface of the high polymer membrane is oxidized to form functional groups such as hydroxyl groups. As an example of REFFET of said system, there is disclosed (in Japanese Patent Publication No. 58-25221) a gate membrane formed of a polyvinyl chloride which is plasticized with dioctyl terephthalate (DOP). It has been reported, however, that such membrane is sensitive to quaternary ammonium ions, etc. Thus, there is a problem in using it as a gate membrane for REFFET (T. Higuchi et al., Analytical Chem., 1970, 1674). Due to aforesaid reasons REFFET has not yet been put into practical use.